In general, the phenomenon of combustion or fire requires three items to be present: fuel, oxidizer and an ignition source. These three items represent the verticies of the so-called “fire triangle”. If any one of these three items is not present in sufficient quantities, combustion cannot take place or be sustained.
Aircraft fuel tank explosions can occur when these three items are present in the ullage space above the liquid fuel in the fuel tank. A fire in the ullage creates a rapid pressure rise (an explosion) in the fuel tank that causes a structural failure of the airframe. In this case, the components of the fire triangle are as follows. The fuel source is the jet fuel vapor that has liberated itself (evaporated) from the liquid fuel, the oxidizer is atmospheric oxygen that is present in the ullage, and the ignition source could be any one of a variety of sufficiently energetic sources such as a spark from an electromechanical component in the fuel tank.
As in-tank fires are rare events indeed, it suggests that while these three items may be present in the fuel tank, the conditions for combustion are not easily satisfied. In fact, the range of air and fuel mixtures that will allow combustion is fairly constrained. Because the amount of oxygen is dependent upon the local atmospheric pressure and the amount of vaporized fuel in the ullage is also dependent upon this pressure as well as the temperature of the fuel, the air-fuel mixture in the ullage space is strongly dependent upon the pressure in the tank and the temperature of the liquid fuel. Consequently, the air-fuel ratios necessary for combustion are met only under certain altitude and fuel temperature conditions.
The challenge of the fuel tank inerting problem is to blanket the ullage space in the fuel tank with an appropriate amount of nitrogen to prevent combustion. Because nitrogen is a spectator in the combustion process, it acts as a diluent to atmospheric oxygen and effectively lowers the flammability of the fuel tank.
One of the means by which an airplane fuel tank may be inerted is to employ a system that uses air separator technology to blanket the ullage space of the fuel tank with nitrogen. These air separators typically employ a hollow-fiber membrane that allows the preferential passage of oxygen. When a bundle of these hollow fibers is exposed to an adequate pressure differential, they will permeate oxygen much more readily than nitrogen. Consequently, it is possible to separate the oxygen from the nitrogen of atmospheric air and pass the then nitrogen-enriched air along to the airplane fuel tank.
The inerting of an airplane fuel tank presents a significant design challenge to provide a desired level of fuel tank safety at the lowest penalty to the airplane. The penalty to the airplane comes in the form of inerting system weight, parasitic losses, and cooling losses. Each of these three elements requires the airplane to burn more fuel and/or carry less payload. The operators of these aircraft desire a system that provides necessary inertness at the lowest cost and weight.
The air separation module (ASM) is one of the most significant elements of an aircraft inerting system. The cost and weight of the ASM is a key driver in a decision to implement an inerting system on an aircraft. Current technology utilizes the external housing of the ASM as the structural member for translating the loads, which can be substantial, from the ASM to the aircraft structure. Due to the dimensional variations of producing a hollow membrane fiber bundle, the ASM housing becomes more complex driving both weight and cost into the overall ASM design. The current methods for addressing the large tolerance accumulations, thermal growth, pressure containment and aircraft vibration environments have negatively impacted the weight and cost of the ASM.